The present invention relates to a stray field detecting method, a stray field detector, and a stray-field-resistant magnetic disk drive in the data read/write device that magnetically reads a signal written in the thickness direction of a recording medium.
The conventional recording technique will be described with a perpendicular magnetic disk drive as an example referring to FIG. 14 and FIG. 15. The perpendicular magnetic disk drive is a magnetic disk drive that possesses a perpendicular magnetic recording medium, a read/write head and so forth, and performs reading/writing by the perpendicular magnetic recording system. The perpendicular magnetic recording system, unlike the conventional longitudinal recording system, forms magnetizations recorded in the thickness direction of a recording medium. In FIG. 14, a perpendicular magnetic recording medium 10 includes a recording layer 101 having the magnetic anisotropy in the thickness direction. The perpendicular magnetic recording medium 10 includes the following types: a single layer magnetic recording medium having this recording layer 101 only, and a multi layer perpendicular disk having a soft under layer 102 between the recording layer 101 and a substrate 103 (hereunder, the magnetic recording medium denotes the multi layer perpendicular disk, unless otherwise specified). The recording layer 101 is made of a perpendicular magnetic film having a high magnetic coercive force for retaining data, and the soft under layer 102 has the characteristic that produces magnetizations when a magnetic field d is applied thereto, but restores the non-magnetized state when the magnetic field is removed therefrom.
A read/write head 140 uses a read/write separate type that generally possesses a read head and a write head separately. The write head includes the following types: a ring head that has a gap on a ring-formed part also used in the longitudinal recording system, and performs writing by a magnetic field leaking from the gap, and an SPT (Single Pole Type) head that has a main pole and an return pole (hereunder, the write head denotes the SPT head, unless otherwise specified). For the read head is used a Magneto-resistive head 144 that can detect the variation of the field as the variation of the resistance (hereunder, called MR head).
Next, the write operation will be outlined with an example in which the SPT (Single Pole Type) head 140 and the multi layer perpendicular disk 10 are assembled. The main pole 141 is magnetized by a magnetic field generated by flowing a current through a coil 143 wound on the upper part of the write head. Since the area of the main pole end facing to the recording medium is very small, the magnetic fluxes are converged to generate a huge magnetic field right under the main pole, and thereby magnetizations are recorded in the recording layer 101. The magnetic field penetrating the recording layer 101 magnetizes the soft under layer 102. On the other hand, the return pole 142 is magnetized in the reverse direction to the main pole 141 by the magnetic field generated by the coil 143, but the area of the end of the return pole is large, and the magnetic field generated is too small to be recorded. However, since a magnetic field in the reverse direction to the main pole is generated in the soft under layer 102 right under the return pole 142, a magnetic field is formed which connects an area right under the main pole 141 and an area right under the return pole 142. In this case, the recording magnetic field is shown by a path 50 starting from the main pole 141 returning to the return pole 142 by way of the soft under layer 102.
As shown in FIG. 15, when a magnetic field 51 is applied perpendicularly to the face of such a recording medium from the outside, the magnetic field penetrates the main pole and the return pole. Now, since the main pole 141 converges a magnetic field on the end (511) of the magnetic pole because of its structure, there is a possibility of generating such an intense magnetic field as demagnetizing magnetizations recorded in the recording layer 101, even if the applied magnetic field 51 is low. This is a phenomenon called the demagnetization or degaussing of recorded magnetizations by the stray field. If this phenomenon occurs, the written information will be lost, and besides in a most remarkable case, even the servo mark and the track and sector identifying signal will be lost, which can be a hindrance to the operation itself as the disk drive (HDD).
The problems that appear when the stray field is applied perpendicularly to a recording medium have been known since. For example, JP-A No. 225901/1995 (patent document 1) and JP-A No. 225526/1993 (patent document 2) disclose the problems and the measures. The patent document 2 discloses a technique that installs a magnetic sensor in a device to thereby detect the magnetic field, and escapes the head outside the data area based on the detection. JP-A No. 100141/2002 (patent document 3) discloses a technique that detects the stray field by means of the read head, reads a pattern for detecting the stray field written on a recording medium by means of the read head, amplifies by a preamplifier using the conventional reading means, performs the A/D conversion, and then detects the stray field by using the average of the amplitude. JP-A No. 272331/2003 (patent document 4) discloses a technique that detects the stray field without flying the head above the magnetic recording medium. The technique according to the patent document 4 measures the resistances of the MR elements each in the state that the stray field is not applied, such as on production, and stores them in a FROM. This technique reads out the values of the FROM from the resistances of the MR elements and the ambient temperature during an actual operation by a temperature sensor, and thereby detects the stray field. It is also conceivable to add a magnetic shielding as the measure against the stray field. JP-A No. 77266/2003 (patent document 5) discloses a technique concerning the magnetic shielding.